1. Field of the Invention
This invention relates to single lens reflex cameras, and, more particularly, to sharp focus detecting arrangements in such camera which enables a photo-electric sensor to produce an electrical output signal commensurate with focus at an improved sensitivity and reliability.
2. Description of the Prior Art
With an SLR (single lens reflex) camera, in focusing a photo-taking lens, it has been the very common practice for almost any photographer to look through the viewfinder for observation of the degree of sharpness of an image of an object formed on the focusing screen by the taking lens, in other words, to depend upon the critical sense of his naked eye. Consequently, such a screen image-dependent focusing procedure is very troublesome as will be perceived by most beginners in using an SLR camera. The beginner suffers considerably from this painstaking experience until he becomes able to adjust the focus of the image accurately. For general amateur users, therefore, an accurate and quick focusing adjustment of the photo-taking lens is very difficult to achieve.
In order to obviate such an inconvenience, many efforts have been devoted to automatization of the focusing procedure. In the case of the SLR cameras, however, any of the proposed methods and apparatus is not amenable to practical production techniques and, when rendered compatible thereto, leads to the production of extremely high priced cameras. Among such problems, it is very serious that the heretofore attained level of accuracy in detecting a condition of sharp focus is not as high as desired. As a result, the conventional methods and apparatus for automatization of the focusing procedure in SLR cameras have not been employed to the same extent as compared with those for automatization of exposure measurement.
It is known to provide a variety of automatic sharp focus detecting systems. Of these, there exists what may be called TTL type system because of the analogy to the TTL type light metering system in that the light entering through the lens is partly utilized to photo-electrically detect a condition of sharpest focus as the photo-taking lens is shifted. This TTL type focus detecting system is, among other type systems, comparatively suited for incorporation in the SLR camera, particularly because the compactness of the camera structure itself can be preserved to a better extent.
Unlike the TTL type light metering system, however, the TTL type focus detecting system must be designed by due consideration for selection of a position which the photo-electric detecting means is intended to occupy and also for establishment of a focus detecting light arrangement rendering it possible for the photo-electric detecting means to operate with high accuracy and reliability. In other words, while the design feature of the light metering system is not confined to the limitation of the position which the photo-electric detecting means occupies, provided that it can receive light entering through the photo taking lens to detect the object brightness, the deisng feature for the focus detecting system is characterized in that the photo-electric detecting means be responsive to the perfect definition of the object image formed by the photo-taking lens. Thus, the availability of the position which the detecting means is intended to occupy is not so much sufficient as desired. This tends to lead to an increase in the complexity of the camera optical system itself and also to a decrease in the accuracy of sharp focus detection due to the fact that it is made more difficult to achieve fine adjustment of the optical paths for the purpose of varying the position of the plane of sharp focus for the focus detecting optical system accurately as the function of the position of the plane of sharp focus for the photo-taking lens. In the extreme case, it is difficult to secure the compact structure of the camera itself.
To avoid the above-mentioned drawbacks, it is desirable to establish the focus detecting light arrangement by utilizing the viewing mirrow of the camera with modification by the provision of a mirror coating that is partly lightpermeable so as to enable the mirror to serve as a beam divider in combination with an auxiliary mirror positioned behind the viewing mirrow to deflect an image-carrying light beam passing through the viewing mirror towards the detecting means. This embodiment is, because of its structure being simple enough and of the capability of fine adjustment of the optical paths with ease, comparatively advantageous except for the problem that as the total amount of incident light on the image receiving surface of the photo-electric detecting means is reduced, the decreased level of accuracy of sharp focus detection results.
In such a TTL type automatic sharp focus detecting system, therefore, further improvements must be directed to increase the accuracy of sharp focus detection to as high a level as desired regardless of the reduction of the total incident light on the photo-electric detecting means.